Method of manufacturing an automobile suspension part

ABSTRACT

For manufacturing an aluminum alloy automobile suspension part, an ingot material is heat treated and forged, a through-hole is formed therein and it is subjected to thermal refining. A minimum length between a rib end on a side of the through-hole and a through-hole end is made 6 mm or larger.

The present application is a divisional application of U.S. patentapplication Ser. No. 13/043,011, filed on Mar. 8, 2011, the entirecontent of which is incorporated herein by reference and claims thebenefit of priority from Japanese Patent Application No. 2010-058167,filed on Mar. 15, 2010.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an automobile suspension part having athrough-hole for weight saving and a manufacturing method thereof.

Description of the Related Art

Conventionally, 6000 series (JIS 6000 series) aluminum alloys such as6106, 6111, 6003, 6151, 6061, 6N01 and 6063 aluminum alloys have beengenerally used for an automobile suspension part. Forging materials madeof such 6000 series aluminum alloys have a high strength, high fracturetoughness and relatively excellent corrosion resistance. The 6000 seriesaluminum alloys themselves consist of a fewer metal elements andtherefore their scrap is easily reused as fused raw materials for 6000series aluminum alloy, and therefore, the 6000 series aluminum alloysare excellent in recycling properties (for example, refer to JapaneseUnexamined Patent Publication No. 2008-163445 and Japanese UnexaminedPatent Publication No. 2008-223108).

Use of such 6000 series aluminum alloys can achieve higher strength ofautomobiles. Meanwhile, in recent years, the problem of warming causedby CO₂ exhausted by automobiles has become more serious. In order toreduce CO₂ exhausted by automobiles, weight saving of automobiles iseffective.

SUMMARY OF THE INVENTION

In automobile suspension parts, it is important to achieve weight savingas well as to ensure strength. A through-hole has been formed on theconventional automobile suspension parts for weight saving. However, asa result of research by the present inventors, it is recognized that theformation of the through-hole generates a coarse crystal grain in aregion in the vicinity of an inner side of a rib (a side where thethrough-hole is formed), thereby lowering the yield strength of thisregion, in turn, the strength of the automobile suspension part.

In consideration of the above problem, an object of the presentinvention is to provide a lightweight automobile suspension part havinghigh strength and a manufacturing method thereof.

To achieve the above object, an automobile suspension part according tothe present invention (hereinafter referred to as suspension part asnecessary) is an aluminum alloy automobile suspension part having athrough-hole, and has a yield strength of a normal section where acrystal grain is not coarsened being 270 MPa or larger and a minimumlength between a rib end as a predetermined region on a side of thethrough-hole in a rib and a through-hole end as a predetermined regionon a side of the through-hole in a web being 6 mm or larger.

With such configuration, the through-hole enables weight saving of thesuspension part and the yield strength of the normal section being 270MPa or larger results in high yield strength of the suspension part. Inaddition, since the minimum length between the rib end and thethrough-hole end is 6 mm or larger, no recrystallized section as aregion where the crystal grain is coarsened occurs in the rib,preventing the strength of the suspension part from lowering.

A manufacturing method of the automobile suspension part according tothe present invention is a manufacturing method of an aluminum alloyautomobile suspension part having a through-hole, the automobilesuspension part having a yield strength of a normal section as a regionwhere a crystal grain is not coarsened being 270 MPa or larger, themethod including an ingot material preparing step of preparing an ingotmaterial, a heat treatment step of performing heat treatment of theingot material, a shaping step of shaping the heat-treated ingotmaterial to form a shaped member, a forging step of forging the shapedmember, a through-hole forming step of forming a through-hole on theforged shaped member, and a thermal refining step of performing thermalrefining of the forged shaped member on which the through-hole isformed, wherein in by forming a through-hole forming region as a regionwhere the through-hole is formed in the forging step and removing thethrough-hole forming region in the through-hole forming step, theautomobile suspension part is made so that a minimum length between arib end as a predetermined region on a side of the through-hole in a riband a through-hole end as a predetermined region on a side of thethrough-hole in a web is 6 mm or larger.

According to the manufacturing method, by forming the through-holeforming region in the forging step and removing the through-hole formingregion in the through-hole forming step so that the minimum lengthbetween the rib end and the through-hole end is 6 mm or larger, weightsaving of the suspension part can be achieved, and the suspension parthaving high strength, in which no recrystallized section occurs in therib, can be manufactured.

The automobile suspension part according to the present inventionenables weight saving through formation of the through-hole as well asreduction of CO₂ exhausted by automobiles. Even when the through-hole isformed, the automobile suspension part has excellent strength.

According to the manufacturing method of the automobile suspension partof the present invention, a lightweight automobile suspension parthaving high strength can be manufactured.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic diagram showing an automobile suspension partaccording to the present invention and FIG. 1B is a schematic sectionaldiagram taken along an X-X line in FIG. 1A for illustrating a minimumlength between a rib end and a through-hole end;

FIGS. 2A and 2B are schematic diagrams for illustrating a recrystallizedsection of the automobile suspension part;

FIG. 3 is a flowchart of a manufacturing method of the automobilesuspension part according to the present invention;

FIGS. 4A and 4B are explanation diagram showing summary of a trimmingmethod in a through-hole forming step; and

FIG. 5A is a schematic diagram showing a test piece in Examples and FIG.5B is a load—displacement graph showing relationship between loadexerted on a test piece and displacement in Examples.

DESCRIPTION OF PREFERRED EMBODIMENTS

An automobile suspension part and a manufacturing method of theautomobile suspension part according to the present invention will bedescribed below in detail with reference to figures.

<Automobile Suspension Part>

As shown in FIGS. 1A and 1B, the automobile suspension part according tothe present invention (hereinafter referred to as suspension part asnecessary) 10 is an aluminum alloy automobile suspension part 10 havinga through-hole h. Here, the suspension part 10 refers to, for example, asuspension arm (L-shape), a lower arm, an upper arm, a knuckle and alink (I-shape) and FIG. 1A shows an upper arm.

In the suspension part 10, the yield strength of a normal section wherethe crystal grain is NOT coarsened (refer to FIG. 2) is 270 MPa orlarger and a minimum length between a rib end as a predetermined regionon a side of the through-hole h in a rib 11 and a through-hole end as aas a predetermined region on a side of the through-hole h in a web 12(hereinafter referred to as minimum length as necessary) being 6 mm orlarger. Configuration of each component will be described later.

[Aluminum Alloy]

As a material for the suspension part 10, aluminum alloys, inparticular, 6000 series (JIS 6000 series) aluminum alloys having highstrength, high fracture toughness and relatively excellent corrosionresistance are generally used. The 6000 series aluminum alloysthemselves consist of a fewer metal elements and therefore their scrapis easily reused as 6000 series aluminum alloy fused materials.Accordingly, the 6000 series aluminum alloys are excellent in recyclingproperties. As 6000 series aluminum alloys, for instance, 6106, 6111,6003, 6151, 6061, 6N01, 6063 are cited.

[Yield Strength of Normal Section of 270 MPa or Larger]

The normal section according to the present invention refers to a regionwhere the crystal grain is not coarsened. In a manufacturing process ofthe suspension part 10, in order to perform shearing for formation ofthe through-hole h, it is required to make a through-hole forming regionas a region where a predetermined scope for forming the through-hole h,that is, a region where a hole is formed, to be thin to form thethrough-hole h. When the through-hole forming region is thick, a burr 13is torn and thus, the shape of the inner side (the side of thethrough-hole h) of the web 12 becomes defective.

Specifically, for example, a shaped member having an outer diameter of70 to 80 mm is made thinner to be 5 to 30 mm in thickness, and then, aregion where a hole is to be formed is made thinner to be 1 to 3 mm inthickness to form the through-hole forming region. When the through-holeforming region of the shaped member is made thin, a large forgingdeformation occurs in the vicinity of the burr 13 and the web 12. Then,when the through-hole formed member where the through-hole h is formedis subjected to heat treatment, heat is applied to a region having muchdeformation, resulting in that a crystal grain is recrystalized in thevicinity of the burr 13 and the web 12, thereby coarsening the crystalgrain.

A region where the crystal grain is not coarsened due torecrystalization is referred to as the normal section, and for example,a portion of a rib 11 (rib portion) and a part of the web 12 (portion ofthe web 12 other than the vicinity of the burr 13) correspond to thenormal section (refer to FIG. 2A). The region where the crystal grain isnot coarsened refers to a region having a recrystalization ratio of 10%or smaller.

Then, the yield strength of the normal section is set to 270 MPa orlarger. By setting the yield strength of the normal section to 270 MPaor larger, the suspension part 10 has excellent strength.

The yield strength can be measured, for example, according to JIS Z 2241and the recrystalization ratio can be measured by means of a sectionalmicroscopic observation (optical microscope).

[Minimum Length between Rib End and Through-Hole End: 6 mm or Larger]

The rib end according to the present invention refers to a predeterminedregion on the side of the through-hole h in the rib 11 of the suspensionpart 10, that is, an end of the inner side of the rib 11 (the side ofthe web 12) and the through-hole end refers to a predetermined region ofthe web 12 on the side of the through-hole h in the web 12 of thesuspension part 10, that is, an end of the inner side of the web 12 (theside of the through-hole h). According to the present invention, moreparticularly, the rib end and the through-hole end are defined asfollows.

As shown in FIG. 1B, a straight line (horizontal) A1 is drawn along ahorizontal side surface of the rib 11 and a straight line A2 is drawnalong a vertical side surface of the inner side of the rib 11. Using anintersection of the straight line A1 and the straight line A2 as anaxis, a line is drawn perpendicular to the straight line A1 and a regionwhere this line overlaps the rib 11 is defined as the rib end. Astraight line (horizontal) B1 is drawn along a horizontal side surfaceof the web 12 and a straight line B2 is drawn along a vertical sidesurface of the inner side of the web 12. Using an intersection of thestraight line B1 and the straight line B2 as an axis, a line is drawnperpendicular to the straight line B1 and a region where this lineoverlaps the web 12 is defined as the through-hole end. In the case of amechanically pressed product (machine press product), an inclination ofthe straight line A2 is assumed to be 1 to 5 degrees that is necessaryfor releasing the forged member from a mold.

As shown in FIG. 1B, the surface shape of the suspension part 10slightly varies in size on the front side (upper side in this figure)and on the back side (lower side in this figure) of the suspension part10. However, in defining the rib end and the through-hole end accordingto the present invention, a smaller distance between the rib end and thethrough-hole end, that is, a minimum distance is used as a reference.

The minimum length d between the rib end and the through-hole end is setto 6 mm or larger. In the suspension part 10, the through-hole h isformed for weight saving. As shown in FIG. 2A, when the through-hole his away from the rib 11 (that is, the minimum length d is 6 mm orlarger), a region where the crystal grain is coarsened byrecrystalization (hereinafter referred to as recrystallized section asnecessary) g does not occur in the rib 11, while as shown in FIG. 2B,when the through-hole h is closer to the rib 11 in the suspension part10 p (that is, the minimum length d is less than 6 mm), the formation ofthe through-hole h causes the recrystallized section g in the vicinityof a region in the vicinity of the inner side of the rib 11.

Since the recrystallized section g has a smaller yield strength than thenormal section, when the recrystallized section g occurs in the rib 11,the yield strength of the region lowers, thereby decreasing the strengthof the suspension part 100. In order to prevent such phenomenon, it isrequired to set the minimum length d to 6 mm or larger. When the minimumlength d is 6 mm or larger, no recrystallized section g occurs in therib 11.

<Others>

Since the effect of weight saving is small when the rib 11 is short andshaping is difficult when the rib 11 is tall, the height of the rib 11is preferably 10 to 70 mm. Since shaping is difficult when the web 12 isthin and the effect of weight saving is small when the web 12 is thick,the thickness of the web 12 is preferably, 5 to 10 mm. In order toprevent defects such as tearing in shearing and to prevent excessivedeforming loads, the thickness of the burr 13 (region where thethrough-hole is formed) is preferably, 1 to 3 mm. To prevent defects dueto forging failure, a curvature radius R of an inclined plane on theside of the straight line A2 is preferably, 2 to 15 mm.

It is noted that the thickness of the web 12 and the curvature radius Rare unrelated to the yield strength of the recrystallized section g inthe rib 11.

<Manufacturing Method of Automobile Suspension Part>

As shown in FIG. 3, the manufacturing method of the automobilesuspension part according to the present invention (hereinafter referredto as manufacturing method of automobile suspension part as necessary)is a manufacturing method of an aluminum alloy automobile suspensionpart having a through-hole, the automobile suspension part having theyield strength of the normal section as a region where the crystal grainis not coarsened being 270 MPa or larger, the method including an ingotmaterial preparing step S1, a heat treatment step S2, a shaping step S3,a forging step S4, a through-hole forming step S5 and a thermal refiningstep S6. Each step will be described below. The aluminum alloy, theyield strength and the through-hole are the same as those described withrespect to the suspension part, and thus, description thereof isomitted.

<Ingot Material Preparing Step>

The ingot material preparing step S1 is a step of preparing an ingotmaterial. The ingot material preparing step S1 includes a fusing andcasting step of fusing and casting an aluminum alloy to prepare an ingotand a cutting step of cutting the ingot to a predetermined length.

In the fusing and casting step, for example, a long round-bar shapedingot of a diameter of 70 mm is made of a liquid containing fusedaluminum alloy having a predetermined composition. The method of fusingand casting the aluminum alloy is not specifically limited and mayemploy any conventionally known method. For example, the aluminum alloycan be fused in a vacuum induction furnace and casted according to acontinuous casting method or a semi-continuous casting method.

In the cutting step, an ingot (long round-bar shaped ingot) is cut witha cutter to be an ingot material (round-bar shaped member) having aratio of length to diameter of 3 or larger. Prior to cutting, the ingotmay be peeled.

<Heat Treatment Step>

The heat treatment step S2 is a step of performing heat treatment(homogenizing heat treatment) of the ingot material. By performing theheat treatment of the ingot material, an intermetallic compoundcrystallized during casting is diffused and a solid solution is formed,resulting in that configuration is homogenized. The heat treatment maybe performed in a furnace according to ordinary conditions. Byperforming the heat treatment in such conditions, sufficienthomogenization can be achieved. For the heat treatment, an air furnace,an induction furnace, a nitre furnace or the like is used according tocircumstances.

<Shaping Step>

The shaping step S3 is a step of shaping the heat-treated ingot materialto form a shaped member. For instance, roll forming and bending arecited as shaping. In roll forming, for example, the ingot material isshaped to a stepped roll formed member by use of a roll forming device.Here, in the case of preparing a shaped member other than an I-shapedmember, such as an L-shaped suspension arm, bending is performed afterroll forming. For example, bending can be performed by use of a bendingmold provided in a press mold.

<Forging Step>

The forging step S4 is a step of forging the shaped member. The ingotmaterial (shaped member) cooled to a room temperature after the heattreatment is heated to a forging start temperature again. Then, theshaped member is hot-forged through forging by mechanical press,hydraulic press or the like to form a final product shape of thesuspension part.

This forging includes, for example, primary forging as rough forging,secondary forging as intermediate forging and finish forging. Primaryforging is a step of pressing the shaped member with a mold for primaryforging and through this step, a primary press member is shaped.Secondary forging is a step of pressing the primary press member with amold for secondary forging and through this step, a secondary pressmember is shaped. Finish forging is a step of pressing the secondarypress member with a mold for finish forging and through this step, theforged member is manufactured. These primary forging, secondary forgingand finish forging are continuously performed without being interruptedby reheating. For example, forging may be performed under the forgingstarting temperature of 400 to 550° C.

In the forging step S4, the through-hole forming region as a regionwhere the through-hole is formed is formed. The thickness of thethrough-hole forming region is set to 1 to 3 mm, for example. Byadjusting the extent of the through-hole forming region through forging,a suspension part having the minimum length of 6 mm or larger can beobtained after punching of the through-hole forming region in thethrough-hole forming step S5. In other words, the position of thethrough-hole end can be adjusted by adjusting the through-hole formingregion.

In the forging step S4, the thickness reduction ratio is 40 to 90%, forexample. Forging is performed so that the part with higher thicknessreduction ratio is located in the inner side of the web (the side wherethe through-hole is formed).

<Through-Hole Forming Step>

The through-hole forming step S5 is a step of forming the through-holeon the forged shaped member. In the through-hole forming step S5, thethrough-hole is formed by removing the through-hole forming region toform the suspension part having the minimum length between the rib endas a predetermined region on the side of the through-hole in the rib andthe through-hole end as a predetermined region on the side of thethrough-hole in the web being 6 mm or larger.

The through-hole can be formed according to any conventionally knownmethod. For example, as shown in FIGS. 4A, 4B, the through-hole 1 c canbe formed on a product part 1 a by means of a trimming device 51 at thesame time as shearing removal of a burr 1 b. In the trimming device 51,(1) a forged member 1 is placed on a trimming die 52 and a punch 57. (2)A punch 55 located above the trimming die 52 is introduced into a hole53 of the trimming die 52 and the forged member 1 is pushed into thehole 53, resulting in that a blade 54 a of an edge part 54 shears andremoves the burr 1 b of the forged member 1. At the same time, a burr 1d is removed to form the through-hole 1 c. In this manner, in thetrimming device 51, one vertical pressing operation of the punches 55,57 can achieve both of shearing (trimming) of the burr 1 b and formationof through-hole 1 c.

<Thermal Refining Step>

The thermal refining step S6 is a step of performing thermal refining ofthe through-hole formed member where the through-hole is formed.Specifically, the forged shaped member on which the through-hole isformed is subjected to solution heat treatment, quenching treatment andage-hardening treatment in this order.

After forging, in order to obtain strength, fracture toughness andcorrosion resistance necessary for the suspension part, thermal refiningtreatment such as T6, T7 is performed. T6 is artificial age-hardeningtreatment to obtain maximum strength after the solution heat treatmentand the quenching treatment. T7 is excessive age-hardening treatmentperformed under conditions exceeding conditions of the artificialage-hardening treatment to obtain maximum strength after the solutionheat treatment and the quenching treatment. That is, the thermalrefining step S6 includes the solution heat treatment step, thequenching treatment step and the age-hardening treatment step.

The solution heat treatment may be performed under ordinary conditions,for example, under a condition in which the member is held in atemperature range of 520 to 570° C. for 1 to 7 hours. For the solutionheat treatment, an air furnace, an induction furnace, a nitre furnace orthe like is used according to circumstances. The quenching treatment maybe also performed under ordinary conditions.

In the age-hardening treatment, conditions of the thermal refiningtreatment such as T6, T7 may be selected from ordinary conditions, forexample, a temperature range of 170 to 220° C. and holding time of 3 to6 hours. For the age-hardening treatment, an air furnace, an inductionfurnace, an oil bath or the like is used according to circumstances.

In carrying out the present invention, another step may be insertedbetween the above-mentioned steps or before or after the above-mentionedsteps so as not to have an adverse effect on the steps. For example, aforeign matter removing step of removing foreign matters such as wastesmay be included, and before or after the thermal refining step S6, amachining step and a surface treatment step of performing machining andsurface treatment of the member, which are necessary for the suspensionpart may be included.

EXAMPLES

The embodiment for carrying out the present invention has been describedabove. Examples for confirming effects of the present invention will bedescribed as compared to Comparative examples that do not satisfyrequirements of the present invention. The present invention is notlimited to these examples.

<Preparation of Test Piece>

First, an aluminum alloy ingot having the same composition as a JIS 6061alloy was fused and continuously casted to prepare an ingot material.This ingot material was subjected to the homogenizing heat treatment,and then, subjected to roll forming and bending to form shaped members.Next, these shaped members are heated and hot-forged, and then, theshaped members, excepting a part of them are formed with a through-hole.After that, the shaped members are subjected sequentially to thesolution heat treatment, the quenching treatment and the artificialage-hardening treatment, as T6 treatment, to form a test piece having inall regions the yield strength shown in FIG. 5A being 270 MPa or larger.

The minimum length d between the rib end and the through-hole end wasadjusted to have a value shown in Table 1.

<Estimation Method>

Mass of the test piece was measured, and strength of the test piece,yield strength of the normal section, yield strength of therecrystallized section and the recrystalization ratio of the rib werealso measured.

[Mass]

The mass was calculated as relative mass assuming that the mass of thetest piece having no through-hole (Comparative example 3) is 100. It isdetermined that a test piece having a value smaller 100 is reduced inweight.

[Test Piece Strength]

The test piece strength was calculated by performing a strength test offixing a bush part of the test piece and applying a load to a ball jointpart, and measuring yield strength. FIG. 5B is a load-displacement graphshowing relationship between the load exerted on the test piece anddisplacement.

The test piece strength was calculated assuming that the test piecestrength of the test piece having no through-hole (Comparative example3) is 100. Then, it is determined that a test piece having a valuesmaller 100 has excellent material strength.

[Yield Strength]

As the yield strength, the yield strength of the normal section and theyield strength of the recrystallized section were measured according toJIS Z 2241. Then, the yield strength was calculated as relative strengthassuming that the yield strength of the normal section of the test piecehaving no through-hole (Comparative example 3) is 100.

[Recrystalization Ratio]

As the recrystalization ratio, the recrystalization ratio of the rib wasmeasured by means of sectional microscopic observation (opticalmicroscope). The results are shown in Table 1.

TABLE 1 Yield Yield strength strength Test piece (normal (recrystallizedRecrystalization Minimum Mass strength section) section) ratio length d(relative (relative (relative (relative (rib) (mm) mass) strength)strength) strength) (%) Example 1 6 93 100 100 92 10 Example 2 8 93.5100 100 92 10 Example 3 10 93.9 100 100 92 10 Comparative 2 92 98.7 10092 20 example 1 Comparative 4 92.5 99.3 100 92 15 example 2 ComparativeNo hole 100 100 100 92 10 example 3 d: minimum length between rib endand through hole end

As shown in Table 1, since Examples 1 to 3 satisfied the scope of thepresent invention, the test pieces in Examples 1 to 3 each had smallermass than the test piece where no through-hole was formed (Comparativeexample 3), thereby achieving weight saving. Further, the test piecestrength was equivalent to that in Comparative example 3, and hadexcellent material strength. In addition, the yield strength wasexcellent and the recrystalization ratio of the rib was also lower thanthat in Comparative examples 1, 2.

On the other hand, since Comparative examples 1 to 3 did not satisfy thescope of the present invention, following results were obtained.

Since the minimum length (d) between the rib end and the through-holeend in Comparative examples 1, 2 did not satisfy the scope of thepresent invention, the test piece strength was poor.

Since the through-hole was not formed in Comparative example 3, the masswas large and thus, weight saving could not achieved.

Like the 6061 alloy, even alloys other than 6061 (ex. 6106, 6111, 6003,6151, 6N01, 6063), as long as they satisfied the configuration of thepresent invention, could obtain more excellent results than alloys thatdid not satisfy the configuration of the present invention.

Although the present invention has been described in detail withreference to the embodiment and the examples, the subject matter of thepresent invention is not limited to the above-mentioned matter and scopeof right of this invention must be widely interpreted based ondescription in claims. As a matter of course, contents of the presentinvention can be widely modified and changed based on the description.

What is claimed is:
 1. A manufacturing method of an aluminum alloyautomobile suspension part having a through-hole, a rib and a webconnecting the rib and the through-hole, wherein a normal section of theautomobile suspension part that includes the rib and whose crystal grainis not coarsened has a yield strength of 270 MPa or larger and arecrystallization ratio of ≦10%, the method comprising: an ingotmaterial preparing step of preparing an ingot material; a heat treatmentstep of performing heat treatment of the ingot material; a shaping stepof shaping the heat-treated ingot material to form a shaped member; aforging step of forging the shaped member; a through-hole forming stepof forming a through-hole in the forged shaped member; and a thermalrefining step of performing thermal refining of the forged shaped memberon which the through-hole is formed, wherein a minimum length of the webbetween a rib end on a side of the through-hole and a through-hole endis 6 mm or larger.
 2. The manufacturing method according to claim 1,wherein the aluminum alloy is a 6000 series aluminum alloy.